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A. NAGODE et al.: STRUCTURAL AND THERMODYNAMIC ANALYSIS OF WHISKERS ON THE SURFACE...
Figure 2 Surface of grey cast iron covered with whiskers; SEI
Figure 3 Typical oxide pattern after black oxide coating; SEI
trast) and the inner (dark contrast) oxide layers However,
a detailed EDXS analysis (Table 1) shows that
whiskers as well as the upper part of the outer oxide
layer is composed of pure Fe-oxide, while in the lower
oxide layer EDXS analysis indicates FeMn-oxides. The
EDXS analysis of the inner oxide layer (dark contrast)
confi rms the presence of Fe, Mn and Cr; however, the
increased content of Si indicates that the inner layer is
composed of Si-rich Fe(MnCr) oxide.
In Figure 4 the internal oxidation of the matrix
around the graphite fl akes near the surface can also be
seen. The internal oxidation actually consists of iron
oxidation (xFe + yO → Fe x O y ) as well as graphite oxidation
(2C + O 2 → 2CO or C + O 2 → CO 2 ). The formation
and growth of the Fe-oxide in the subsurface is
promoted by oxygen penetration through the graphite
boundaries with the metallic matrix [15, 16]. In order to
show the element distribution on the cross section of the
sample covered with whiskers an energy dispersive xray
spectroscopy (EDXS) maps are presented. EDXS
maps confi rm above mentioned results of EDXS quantitative
analysis. Distribution of carbon shows graphite
fl akes at the bottom of the image, however, at the top of
the image the signal of C shows the signal C, Kα x-rays
which come from the bakelite that the sample for metallographic
preparation has been put in. The x-ray diffrac-
Figure 4 Cross section of the sample covered with whiskers;
BEI
tion analysis was used for a phase identifi cation of the
surface covered with whiskers as well as for the surface
without them. The analysis showed that the only difference
between two XRD patterns lies in the ratio of the
peak intensities between the hematite (Fe 2 O 3 ) and the
magnetite (Fe 3 O 4 ). Namely, a detail from the XRD spectrum
(2 theta = 32 ° - 2 theta = 36 °) of the surface covered
with whiskers shows a higher peak-intensity ratio
between the hematite (Fe 2 O 3 ) and the magnetite (Fe 3 O 4 )
(Figure 6a) in comparison to the x-ray diffraction pattern
of the surface without whiskers (Figure 6b). Since
the surface with brown stains is covered with whiskers
this result indicates that the whiskers are composed of
hematite (Fe 2 O 3 ).
THERMODYNAMIC CALCULATIONS
For the calculation of particular phase diagram, Figure
7, the data for pure elements were taken from Dinsdale
[17], for the substances data from Ansara [18].
Thermodynamic assessments on the Fe-O phase dia-
Fe, Kα Mn, Kα
Cr, Kα Si, Kα
O, Kα C, Kα
Figure 5 EDXS maps of Fe, Mn, Cr, Si O and C
12 METALURGIJA 52 (2013) 1, 11-14